In one particularly heated exchange, Democratic Rep. Luis Gutierrez of Illinois called the DHS chief a liar after she defended the administration policy of separating migrant children from their parents at the U.S.-Mexico border.

"It is as if you can’t see the reality of modern immigration or contributions of anyone who came from countries other than Norway or other parts of Europe. It’s as if you and the Trump administration are blind," he said. "Shame on everybody that separates children and allows them to stay at the other side of the border fearing death, fearing hunger, fearing sickness. Shame on us for wearing our badge of Christianity during Christmas for allowing the secretary to come here and lie."

Nielsen has been under intense scrutiny from lawmakers and the public over the past months for enforcing the administration's staunch anti-immigration policies. President Donald Trump has also reportedly expressed displeasure with her job performance.

At the hearing before the House Judiciary Committee on Thursday, however, Nielsen firmly defended the administration's practices by insisting, as she has in the past, that no such family separation policy existed.

She fired back at Gutierrez calling her a liar, saying, "Only to say that calling me a liar is fighting words. I'm not a liar, we've never had a policy for family separation."

A family separation policy, she said, "would mean that any family that I found at a port of entry I would separate, it would mean that every single family that I found illegally crossing, we would separate. We did none of those."

At least 2,600 children were separated from their parents under the Trump administration's "zero tolerance" policy at the border, which was in effect from May 6 through June 20. Under pressure, Trump signed an executive order in June to end the practice and allow families to be detained together.

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Rep. Hank Johnson, D-Ga., also asked Nielsen about whether the Department of Homeland Security lost track of separated families. She insisted there is a database and “we know where all the children were, we knew where all the parents were."

But this past October, the DHS inspector general found that DHS was "not fully prepared to implement the administration's zero-tolerance policy or to deal with some of its after-effects" and "struggled to provide accurate, complete, reliable data on family separations."

Rep. Jerry Nadler, D-N.Y., the incoming chairman of the House Judiciary Committee, also had sharp words for Nielsen, saying he will conduct rigorous oversight of DHS when Democrats take control of the House next month.

"As we move into the next Congress, I want to put you and the department on notice. The time for accountability has arrived," he said. "Members of this committee have a responsibility to ask hard questions of any administration to ensure that federal agencies are working in accordance with the law and in the interests of the American people. That job begins today."

Nielsen was also pressed by Rep. Sheila Jackson, D-Texas, about the death of 7-year-old Jakelin Caal Maquin, a Guatemalan immigrant who died of dehydration and shock hours after being taken into custody with her father by the U.S. Border Patrol earlier this year.

"Jackie did not have to die. She did not have to die. Jackie did not have to die," Jackson said, questioning why there were not more medical and language services immediately available at the time. "These are human beings!"

Nielsen claimed that Maquin's father did not indicate any health concerns at first, saying CBP revived her twice before they got her to a hospital.

"To put this in perspective, this is exactly why we try to encourage migrants to go a port of entry. Unfortunately, they arrived in the middle of the night in the middle of nowhere," she said. "As soon as [the father] indicated there was a health issue, we did what we could do as quickly as possible to get her to medical care ... the men and women of CBP did everything they could do."

Later, Nielsen was asked about Trump's demand for $5 billion from Congress to build his long-promised border wall. She sidestepped the question of why the president threatened to shut down the government over funding for the wall, despite his repeated insistence that Mexico would foot the bill. She said she does not believe the government should shut down over the wall, but also said she did not disagree with the president's position.

"So, I guess I'm confused. If Mexico is supposed to pay for the border wall, why is the Trump administration and your department of Homeland Security petitioning Congress for $5 billion dollars in American taxpayer money to pay for that same wall?" Rep. Hakeem Jeffries, D-N.Y., asked.

"All I can tell you from my perspective is why we need a wall, how much wall we need, and where we're going to build it," she said.